A prior, commonly-owned patent, U.S. Pat. No. 6,455,505, to Morgan, which is hereby incorporated by reference herein in its entirety, disclosed a color wheel system that recaptured light from the spoke areas—the boundary areas between colors—of a color wheel. The color wheel typically is a disk-shaped assembly of dichroic filters. Other shapes of “color wheels” are possible, such as rotating drums or polygons. A white-light beam is focused on the color wheel and the dichroic filters, which may be transmissive or reflective and which thereupon filter the white light to form a primary-color light beam. The color wheel includes at least one filter “segment” for each of the primary colors, and most likely also includes a white segment.
Spinning the color wheel so that each filter passes through the point at which the white light beam strikes the color wheel generates a sequential primary color light beam. The color wheel typically is spun fast enough to create at least one primary color period for each primary color during each frame of a video image. Spinning the wheel faster, or using multiple filter segments for one or more of the primary colors can reduce color separation artifacts that allow the viewer to detect the sequential color nature of the display system.
While a sequential color display system typically costs less to produce than a simultaneous color display system, images created by a sequential filtered color display system are not as bright as images created by simultaneous color display systems using the same light source. This is because at any given time only a portion of the light generated by the light source is being used to form the image. For example, when using a three-color equal-segment filter wheel, each primary color is produced only ⅓ of the time. Additionally, when a sequential filter such as a color wheel is used, the light during the filter transitions, typically called spoke times, will be a varying mixture of the two filters being changed in and out. The Morgan patent taught that such spoke light can be recaptured and used during white light periods, as it is undesirable to “throw away” light, given that high image brightness is one of the major desires of customers in the projection display market.
While the above approach is described in the context of a color wheel having filters for the primary colors and possibly white, the use of “neutral density” filters has also been disclosed by Hewlett et al. in commonly owned U.S. Pat. No. 5,812,303, which is hereby incorporated by reference herein in its entirety. Such neutral density filters, instead of providing different colors, provide additional segment filters of the same color as the regular primary colors, except with a lower level of reflectivity (or transmissivity for a transmissive application). As disclosed in the Hewlett et al. patent, such neutral density filters allow for the use of longer modulated bit times to accomplish lower intensity color signals, which is sometimes important for proper timing for the modulation of the pixels of a Digital Micromirror Device (DMD) or other light modulation system. Specifically, as discussed in the '303 patent, there may be limitations on the minimum bit times available for the display of bits, which would otherwise pose a limitation on the least significant intensity bits, and the neutral density filters by their lower transmissivities provide for lower intensity light without necessitating ever-shorter “on” times for the pixels. In this type of a wheel, there will be transitions both between color filters, e.g., between red and green filters, and between regular color filters and Neutral Density (“ND”) filters, e.g., between green filter segments and ND green filter segments.